1. Field of the Invention
The present invention relates to a centering apparatus for an information recording disk. It particularly relates to a centering apparatus for performing centering of a disk which is constituted by a pair of circularl substrates each having a signal recording layer.
2. Background of the Invention
In the case of an optical information recording disk such as a video disk or a DRAW (direct read after write) disk, information is recorded in the form of numbers of fine pits by use of a laser light spot. Therefore, it is important to protect the layer on which the pits have been formed, that is, a signal recording layer. Therefore, there has been developed a disk for the system in which a pair of circular substrates each having a signal recording layer are combined in opposition to each other with the respective signal recording layers disposed inside.
FIG. 1 shows a centering apparatus for performing centering of a disk relative to a turntable. As shown in FIG. 1, the centering apparatus includes a centering member 4 provided at its outer circumferential portion with a conical taper portion 1 and which is slidably attached to a driving shaft 3 of a turntable 2. A coil spring 5 is provided between the centering member 4 and the turntable 2. A ring stopper 6 prevents the centering member 4 from coming off from the driving shaft 3. A cuplike pressing member 8 clamps a disk 9 onto the turntable 2. The pressing member 8 is rotatably attached to a support member 12 through a ball 10 and a ball socket 11 so as to be made to come into contact with or separate from the disk 9 mounted on the turntable 2.
As described above, the disk 9 is constituted by a pair of circular substrates 13 and 14 each having a signal recording layer and a central hole. The substrates 13 and 14 are combined with each other with their signal recording layers disposed on the interior.
In the thus arranged centering apparatus, as shown in the drawing, centering of the disk 9 relative to the turn table 2 is performed by causing the taper portion 1 of the centering member 4 to engage with the central hole of the disk 9. In this state, information reproducing on the signal recording layer of the lower one of the circular substrates 13 is performed by a reproducing portion or optical pickup 16. In the same state without reversing the disk 9, information reproducing of information on the signal recording layer of the other or upper circular substrate 14 is performed by another pickup 17.
In such an apparatus, the two pickups 16 and 17 are provided corresponding to the respective signal recording layers of the circular substrates 13 and 14 so as to perform recording and reproducing on the respective signal recording layers without reversing the disk 9. However, in such an apparatus the following problem occurs in the case where the circular substrates 13 and 14 forming the disk 9 as shown in the drawings are combined with a radial displacement relative to each other of a distance e. That is, although reproducing on the signal recording layer of the lower circular substrate 13 abutting on the disk holding surface of the turntable 2 is smoothly performed, it is difficult to smoothly perform the reproducing on the signal recording layer of the other circular substrate 14 because the upper circular substrate 14 is rotated under the condition that the central axis of the circular substrate 14 is displaced by e from the rotary central axis of the turntable 2.